(1) FIeld of the Invention
The present invention relates to a novel polyester polymer which may be used as the raw material for electrical or electronic equipments, parts of machines, fibers, or the like, a process for preparing the same, and a novel diimide-dicarboxylic acid which is a raw material for preparing the polyester polymer. More particularly, the present invention relates to a novel polyester polymer excellent in the mechanical strength and heat resistance, which therefore is useful as various kinds of fibers, films, or engineering plastics.
(2) Description of the Prior Art
In orders to improve the properties of polyethyleneterephthalates, there is proposed in Japanese Laid-open Patent No. 72393/1974 a method of reacting a polyethyleneterephthalate with an acyloxy-aromatic carboxylic acid.
However, the modification by the reaction of the acyloxy-aromatic carboxylic acids has its limit in the improvement of the mechanical strength, and therefore, there is a demand for more excellent modified polyesters.